Replace use of boost::mutex

Since the C++11 standard now has a full array of thread related classes
we no longer need to use the boost equivalent. Switching to the appropriate
std class should allow for easier maintenance.

The static analyzer was complaining about boost::mutex since the analyzer doesn't know it is thread safe. Rather than modify the analyzer I believe it is better to switch to the appropriate C++11 class.
An atomic is much more performant.